FIG. 2 is a sectional front view of a prior art solid electrolytic capacitor 1, and FIG. 1 is a perspective view of a prior art capacitor element 2 (see, for example, Japanese Examined Patent Publication No. HEI4-19695 (1992)).
The solid electrolytic capacitor 1 includes an aluminum case 3 having a top opening, the capacitor element 2 contained in the case 3, and a rubber packing 30 which seals the opening of the case 3. An upper edge portion of the case 3 is curved to fix the packing 30, and a plastic seat plate 31 is attached to the top of the case 3. Lead wires 21, 21 extend from the capacitor element 2 through the packing 30 and the seat plate 31, and then bent laterally.
As shown in FIG. 1, the capacitor element 2 includes an anode foil 4 of an aluminum foil coated with a dielectric oxide film and a cathode foil 5 of an aluminum foil, which are rolled together into a roll with a separator 6 of an insulative material such as paper interposed therebetween and fixed by a tape 26. The roll is impregnated with a solid electrolyte such as a TCNQ (7,7,8,8-tetracyanoquinodimethane) complex salt, or includes an electrically conductive polymer layer provided therein. Lead tabs 25, 25 respectively extend from the anode foil 4 and the cathode foil 5, and the lead wires 21, 21 respectively extend from the lead tabs 25, 25.
Where the electrically conductive polymer layer is to be formed between the foils 4 and 5, the capacitor element 2 is impregnated with a mixture solution containing ethyl alcohol as a diluent, 3,4-ethylenedioxythiophene and iron(III) p-toluenesulfonate, followed by thermal polymerization.
Although the solid electrolytic capacitor 1 having such a construction is widely used, there is a market demand for a capacitor having a smaller size and a greater capacitance. To this end, a capacitor has been proposed whose cathode foil 5 is coated with a metal nitride film (see, for example, Japanese Unexamined Patent Publication No. 2000-114108).
In general, the dielectric oxide film is not intentionally formed on the cathode foil 5, but formed by natural oxidation. Therefore, the capacitance C of the capacitor is equivalent to a capacitance obtained by connecting the capacitance Ca of the anode foil 4 and the capacitance Cc of the cathode foil 5 in series, and represented by the following equation:C=Ca×Cc/(Ca+Cc)=Ca×1/(Ca/Cc+1)
That is, if the cathode foil 5 has the capacitance Cc, the capacitance C of the capacitor is smaller than the capacitance Ca of the anode foil 4. Therefore, a film of a metal nitride such as TiN is formed on the cathode foil 5 by sputtering or vapor deposition, and electrically connected to an aluminum base of the cathode foil 5. Thus, the capacitance of the capacitor can be increased without size increase of the capacitor because the cathode foil 5 has no capacitance. However, the inventor has found that the cathode foil 5 having such a structure is less effective for increasing the capacitance of the solid electrolytic capacitor 1.
Further, it has been proposed that the capacitance Ca of the anode foil 4 is increased by increasing the dielectric constant of the anode foil 4 (see, for example, Japanese Unexamined Patent Publication No. HEI5-121275 (1993)). For preparation of the anode foil 4 having an increased dielectric constant, a surface of a titanium anode base is oxidized in a solution containing an electrolyte thereby to be formed with a dielectric film of titanium oxide. Titanium oxide has a relative dielectric constant of about 100 (no unit), which is higher than that of aluminum oxide. Since the capacitance is proportional to the relative dielectric constant, the capacitance of the anode foil can be increased.
However, the dielectric film formed by the direct oxidization of the surface of the titanium base is liable to be aged over time. That is, it is difficult to form a stable titanium oxide film on the surface of the titanium anode base. Further, a voltage of 10V to 20V at the lowest is required to be applied for the formation of the titanium oxide film on the surface of the titanium base as described in Japanese Unexamined Patent Publication No. HEI5-121275. A voltage permitted to be applied is about triple the rated voltage of the solid electrolytic capacitor 1 to be produced. The inventor has experimentally confirmed that, if the anode is to be coated with titanium oxide, the solid electrolytic capacitor 1 should have a rated voltage of not lower than 6.3V and, therefore, it is impossible to form a titanium oxide film on an anode of a solid electrolytic capacitor having a lower rated voltage on the order of 2V to 3V.